Particulate number emissions and particle micro-characteristics from natural gas engines: the role of lubricating oils

Reducing particulate number (PN) emissions from lubricating oils (L-oil) in natural gas engines is key to meeting the next-stage emission regulations. However, insufficient understanding remains of how the L-oils combustion affects the physicochemical properties of particles in natural gas engines. This study investigated the effects of L-oil composition and viscosity on the emission characteristics of natural gas engines through experiments. TEM-EDX was employed to analyze differences in microstructure and chemical composition between particles generated by different L-oils. The results indicated that, at 1700 r/min and 10 % load, emission performance deteriorated significantly with L-Soil30 (mineral base oil and high-sulfur additives), with PN emissions reaching 2.79 ∗ 10⁷ N/cc. By contrast, the PN emissions of L-oil30 (synthetic base oil) with a similar viscosity grade were only 9.23 % of those of L-Soil30. As L-oil viscosity increased, the emissions of CO, HC and PN all decreased, while the proportion of accumulation mode particles increased. According to TEM-EDX results, a regular spherical ZnO particle was present in soot particles of L-Soil30, which adsorbed carbon particles to form cluster-like particle aggregates. However, the particle morphology of L-oil30 was chain-like. The L-oil50 particles were irregular in morphology, appeared as flocculent aggregates, and had a larger primary particle diameter. Additionally, EDX spectrum showed that as L-oil viscosity increased, the C content in particles increased while O content decreased. Meanwhile, particles from synthetic oils had a higher Si content. This study can provide guidance for reducing PN emissions from natural gas engines.